The human immunodeficiency virus type-1 (HIV-1) encodes four accessory genes that regulate various aspects of the host cell biology. The vpr gene encodes a 96-amino acid protein (Vpr) that causes cell cycle arrest in G2 and apoptosis in the infected lymphocytes. The cellular pathways leading to Vpr-induced arrest and apoptosis can be activated in most mammalian cell types and are therefore, thought to be evolutionarily conserved. Vpr presumably utilizes cellular signaling pathways to induce its cytostatic and cytotoxic effects. Several studies have indicated that the effects of Vpr on the cell are similar to the effects of genotoxic drugs and other DNA damaging agents, such as radiation. However, we previously demonstrated that a major cellular protein involved in the response to genotoxic agents, p53, was not necessary for mediating the effects of Vpr. In the present study, we examined a recently identified DNA damage-signaling protein (the ATM- and Rad3-related protein, ATR) for its potential role in the induction of G2 arrest by Vpr. We show that inhibition of ATR by genetic means or with pharmacological inhibitors abrogates Vpr-induced cell cycle arrest in mammalian cells. ATR is a serine-threonine kinase that becomes activated in response to DNA damage. Activation of ATR typically results in direct phosphorylation of its target, the Chkl kinase. We detected phosphorylation of Chkl in response to Vpr expression. Since Chkl activation results in inactivation of Cdc2, our observations are consistent with the notion that the Vpr-induced cell cycle arrest involves an ATR-dependent stress-response pathway that leads to G2 arrest. By inhibiting ATR, we demonstrated suppression of another function of vpr, transactivation of the viral promoter. These observations may have important ramifications in the area of anti-HIV therapy. The goal of this proposal is to elucidate the specific steps of the DNA-response pathway that are activated by Vpr, and to compare such steps with the physiological activator, DNA damage.